I'm having this exact problem! I have two character sprites, both in the EXACT same folder, BOTH defined correctly, and BOTH within the images folder. Literally nothing is different about the two except their names, but for some reason, when I try to show one, renpy claims that the image doesn't exist. I copy-pasted the file name and everything, so what gives???
EDIT: Okay, this is what happened on MY computer. It's a PC running Windows 10, and it might be a local issue, but check just in case. This was driving me nuts for HOURS.
I went back and double-checked my files, and I noticed that for some reason, windows was AUTOMATICALLY putting a space before the sprite I intended to call, whereas I was trying to call the file name without the space. It didn't do this for any of my other sprite files, which is why they got called normally. When I tried to remove the space, it gave me a message that stated that because the file names were the same, it wouldn't change the file, which means that it wouldn't remove the space. I had to remove the space ALONG with a few other letters, and update the new name in renpy. Although it sounds tedious, doing this ACTUALLY made it work! The sprite now shows up and functions as it should.
Here's my advice: Before trying to call any images, go back into the files and MAKE SURE that they don't have that space at the beginning. In fact, remove every space in the file name by going to the beginning of each word in the file name and hitting Backspace. If you see the text move, that means there was a space there. Replace the spaces where you want them to be, and change the file name slightly so that it saves your changes. Then, if you don't want to update everything in your script, go back and change the file name to what you originally wanted it to be. Or, if you want to avoid all that, do a full copy and paste into renpy and do NOT change the file name AT ALL.